Remove 3 similar items from an array using mongodb - arrays

Question

I have this document and i need remove only 3 items with name "Test" using only 1 request.
{ "_id" : 1, "items" : ["Test, "Test", "Test", "Test", "Test", "Sword", "Sword]}
It must become { "_id" : 1, "items" : ["Test", "Test", "Sword", "Sword]} after request.

Answer 1

Maybe something like this:
db.collection.aggregate([
{
"$unwind": "$items"
},
{
$group: {
_id: "$items",
cnt: {
$sum: 1
},
it: {
$push: "$items"
},
origid: {
"$first": "$_id"
}
}
},
{
$project: {
_id: "$origid",
items: {
$slice: [
"$it",
0,
2
]
}
}
},
{
$group: {
_id: "$_id",
items: {
$push: "$items"
}
}
},
{
$addFields: {
items: {
"$reduce": {
"input": "$items",
"initialValue": [],
"in": {
"$concatArrays": [
"$$this",
"$$value"
]
}
}
}
}
}
])
Explained:
unwind the items array so you can group after
group by item so you get the repeating values
project with slice to remove more then 2x items in array
group to form back the document but without the >2 repeating values
project with concatArrays to concat arrays of items.
Playground

Answer 2

You can use this aggregation to get the desired result. The query uses the $reduce Aggregate Array Operator to iterate over the items array, and discards the first 3 matching "Test" items.
db.collection.aggregate([
{
$set: {
items: {
$reduce: {
input: "$items",
initialValue: { result: [], count: 0 },
in: {
$cond: [ { $and: [ { $eq: [ "$$this", "Test" ] }, { $lt: [ "$$value.count", 3 ] } ] },
{ result: "$$value.result", count: { $add: [ "$$value.count", 1 ] } },
{ result: { $concatArrays: [ "$$value.result", [ "$$this" ] ] }, count: "$$value.count" }
]
}
}
}
}
},
{
$set: {
items: "$items.result"
}
}
])